1. Field of the Invention
The present invention relates generally to the field of magnetic resonance imaging (MRI) and MR spectroscopy, and more specifically, to a technique for accurately measuring R.sub.2 ', the rate constant for reversible dephasing of the transverse magnetization from magnetic field inhomogeneity such as that arising from intravascular changes in the oxidation state of hemoglobin, from brain iron, or from cancellous bone.
2. Description of the Prior Art
The effective nuclear magnetic resonance (NMR) transverse relaxation rate R.sub.2 *(.ident.1/T.sub.2 *) can usually be decomposed into two separate contributions: one arising from irreversible dephasing which occurs with rate constant R.sub.2 (.ident.1/T.sub.2), and one due to reversible dephasing occurring with rate constant R.sub.2 ' (.ident.1/T.sub.2 '). One way to measure R.sub.2 ' is to acquire two images with the same echo time, one of which is obtained with the gradient echo, the second with the spin echo. R.sub.2 ', by definition, can then be directly obtained from the pixel-to-pixel ratio of the two images. However, such an approach inevitably requires two separate scans. Another possible approach to measuring R.sub.2 ', which also requires two separate scans, is described by Wismer et al. in an article entitled "Susceptibility-Induced MR Line Broadening: Applications to Brain Iron Mapping," J. Comput. Assist. Tomogr., Vol. 12, page 259 (1988), and by Majumdar et al. in an article entitled "In Vivo Relationship Between Marrow T.sub.2 * and Trabecular Bone Density Determined With a Chemical Shift-Selective Asymmetric Spin-Echo Sequence," J. Magn. Reson. Imaging, Vol. 2, page 209 (1992). These articles describe an echo offset technique whereby the phase-reversal RF pulse in the spin echo is offset in increments .DELTA..tau. in which case the signal can be shown to evolve as e.sup.-2.DELTA..tau.R2'.
Single scan techniques for measuring R.sub.2 ' are described by Pelc et al. in an article entitled "A Method For Mapping of T.sub.2 ' and For Improved T.sub.2 Measurements," Proc. Magn. Reson. Med., Seventh Annual Meeting San Francisco, Vol. 2, p. 746 (1988) and by the present inventors in an article entitled "Method For Image-Based Measurement of the Reversible and Irreversible Contribution to the Transverse Relaxation Rate," J. Magn. Reson. Ser. B, Vol. 111, pp. 61-69 (1996). These techniques exploit the fact that the signal after application of a 90.degree.-.tau.-180.degree.-.tau. pulse pair evolves with two different relaxation rate constants (R.sub.2 *=R.sub.2 +R.sub.2 ' for the first .tau.-period, and R.sub.2 -=R.sub.2 -R.sub.2 ' for the second .tau.-period) before and after applying the 180.degree. pulse. Unfortunately, such techniques have a very limited sensitity when R.sub.2 ' is much smaller than R.sub.2, in which case R.sub.2 * and R.sub.2 - become almost identical. A method which provides improved sensitivity for derivation of small R.sub.2 ' is desired.
As described by Ordidge et al. in an article entitled "Simultaneous T.sub.2 and T.sub.2 *-Weighted MRI Using a New Form of Stimulated Echo," Proc. Soc. Magn. Reson., Third Annual Meeting Nice, Vol. 1, p. 670 (1995), the T.sub.2 and T.sub.2 *-dependence can also be demonstrated in a modified form of stimulated echoes collected in a single scan. However, the technique described by Ordidge et al. is incompatible with multi-slice acquisition and suffers from inferior signal to noise ratio due to the use of stimulated echoes. A technique for measuring T.sub.2 and T.sub.2 * is desired which does not suffer from these limitations.
In particular, a technique is desired which can generate in a single pass and in multislice mode T.sub.2 * and T.sub.2 -weighted images from which T.sub.2 ' images can be exactly computed with high sensitivity for application in areas such as BOLD (blood's oxygenation level dependent) based functional imaging, assessment of bone quality (trabeculae bone density), characterization of brain iron concentrations, the study of tissue oxygenation in muscle, and perfusion by means of susceptibility agents. The techniques of the invention have been designed to meet these needs in the art.